That's a nice grinder, and you should be able to get good results. Not to sound evasive, but having that grinder gives you a great advantage over specifying angles: it lets you experiment with
your chains, and
your saw, and
your type of cutting, in
your wood! Try different settings and see if you notice a difference (positive or negative), and develop your own 'recipe'!
As
@blades notes above, Oregon recommends a 55° head tilt angle, as well as a 10° 'down angle' on many chains. 25° is often recommended for harder (and frozen!) woods. 35° is often recommended for softer woods. No reason you can't use 27-1/2° if that works for you! Also experiment (carefully) with different depth gauge settings (once you remove more, it's hard to put it back!). Then get back to use and let us know what you find!
As for competing with your uncle, check to see if you are also using the same chains: 'full-chisel' chains will normally cut faster, but dull faster in dirty wood. 'Semi-chisel' cutters will cut slower, but keep going. I use the analogy of high-speed tires and all-season radials: the first are fun, until it rains. Different brands of chains can affect performance too.
But ask him if he is 'square filing': that will often produce longer chips, especially in soft wood, but is a whole thing on its own.
Philbert